At the time, Mr Cook was investigating Mr Rees over the murder of another private investigator, Daniel Morgan, in 1987. The case against Mr Rees collapsed early last year.

At the Leveson Inquiry yesterday, a lawyer for the Mail On Sunday accused Hugh Grant of not checking his facts before “smearing” the newspaper by claiming it had hacked into his phone.

Liz Hartley said the actor was “leading a campaign against the media” and had made “serious allegations” based on “thin” evidence. Miss Hartley, the head of legal services at Associated Newspapers, publisher of the Mail on Sunday and the Daily Mail, rejected several claims made by Mr Grant to the inquiry, including a suggestion that reporters had intercepted his voicemails. In an exchange with Robert Jay QC, counsel to the inquiry, she also said the company stood by its statement that Mr Grant’s evidence contained “mendacious smears”.

The actor took exception to an article in the Mail on Sunday in 2007 which claimed that his relationship with his girlfriend had been harmed by alleged flirtatious phone calls with a “plummy-voiced” film executive in the US. He told the inquiry that the only woman who fitted the bill was an assistant to a friend in the US who rang him and left jokey messages, leading him to conclude that the MoS had listened to his voicemails.

Miss Hartley said the Mail on Sunday had told Mr Grant last summer that the story came from a source close to his then girlfriend, Jemima Khan.

The inquiry also heard from Peter Wright, the editor of the Mail on Sunday, who admitted that his newspaper had used a private investigator, Steve Whittamore, for 18 months after he was charged with using illegal means to obtain information.